1. Field of the Invention
The present invention relates to a secondary battery charge controller including a protection function against overcharge and over-discharge and relates to an effective technique used for example, in a charge controller incorporated in a lithium-ion battery pack and a semiconductor integrated circuit for the charge controller.
2. Description of the Related Art
In secondary batteries such as lithium-ion batteries, overcharge or over-discharge reduces the battery lifetime. Accordingly, the conventional secondary batteries of mobile phones and the like are often composed as battery packs each incorporating a protection semiconductor integrated circuit (hereinafter, referred to as a protection IC) together with a battery cell in a single case, the protection IC having a protection function against overcharge and over discharge.
In the case of using the aforementioned battery pack, the main device is provided with a charge-controlling semiconductor integrated circuit (hereinafter, referred to as a charge control IC) for charging a secondary battery by voltage from a DC current power supply (a charging power supply) such as an AC adaptor.
One of the inventions concerning a charge controller composed of a protection IC and a charge control IC as described above is disclosed in Japanese Patent Laid-open Publication No. 2000-92735 (Japanese Patent No. 4003311), for example. In another proposed invention, a protection IC and a charge control IC are incorporated in a battery pack (Japanese Patent Laid-open Publication No. 2004-296165).
FIG. 9 illustrates a configuration example of a charge controller including: a conventional battery pack incorporating a protection IC; and a charge control IC charging the battery pack.
A battery pack 100 of FIG. 9 includes a protection IC 11′ equipped with a protection function against overcharge or over-discharge. The battery pack 100 includes, in addition, a charge control FET (a field-effect transistor) 13 and a discharge control FET 14, which are provided in series between a terminal P− connected to a charging power supply (an AC adaptor) and a terminal B− on the negative electrode side of the secondary battery 20. The protection IC 11′ is configured to turn off the charge control FET 13 when the battery voltage reaches a predetermined voltage (about 4.275 V in the case of a lithium ion battery) or higher after charging starts.
In the invention described in Japanese Patent Laid-open Publication No. 2000-92735 (Japanese Patent No. 4003311), when the battery voltage reaches a predetermined voltage (about 4.2 V in the case of a lithium ion battery) after charging starts, the charge control IC 30 switches the control from constant-current charge to constant-voltage charge.
In a charge controller including: a charge control IC; and a battery pack incorporating a protection IC as shown in FIG. 9, the charge control IC monitors voltage across terminals P+ and P− of the battery pack 100 for constant-current charge control and constant-voltage charge control. However, in this case, the voltage across the terminals P+ and P− of the battery pack monitored by the charge control IC is higher than voltage across the terminals B+ and B− of the battery cell 20 by the impedances of the charge control FET 13 and the discharge control FET 14 and the like.
Accordingly, if the charge control IC is designed so as to switch from constant-current charge to constant-voltage charge based on the voltage across the terminals P+ and P− of the battery pack 100 when the voltage across the terminals P+ and P− reaches 4.2 V, for example, as shown in FIG. 10A, the switching takes place when the voltage across the terminals B+ and B− of the battery cell 20 is still about 4.15 V (at timing t4). The voltage across the terminals B+ and B− of the battery cell 20 then gradually increases by the constant-voltage charge, and the voltage across the terminals B+ and B− of the battery cell 20 gets close to the voltage across the terminals P+ and P− of the battery pack 100 while the charge current decreases. As shown in FIG. 10B, when the charge current decreases to a predetermined current value (at timing t5), the charge control IC 30 turns off the charging transistor 31 to terminate the charge.
Herein, in characteristic curves of changes in voltage in FIG. 10A, the solid line represents the voltage across the terminals (B+, B−) of the battery cell, and the dashed line represents the voltage across the terminals (P+, P−) of the battery pack.
If the charge control IC is configured to switch from constant-current charge to constant-voltage charge based on the voltage across the terminals P+ and P− of the battery pack 100, as shown in FIG. 10B, timing t4 of switching from constant-current charge (quick charge) to constant-voltage charge comes earlier. Accordingly, constant-current charge period T1 is shortened, and subsequent constant-voltage charge period T2 is lengthened. The total required charge time (t1-t5) is therefore lengthened.
In the invention of Japanese Patent Laid-open Publication No. 2000-92735 (Japanese Patent No. 4003311) described above, the protection IC 11′ monitors the voltage across the terminals B+ and B− of the battery cell 20 and, when the cell voltage reaches a predetermined voltage, sends a signal to the charge control IC 30 from the protection IC 11′ for switching from constant-current charge to constant-voltage charge. In such a case, however, it is necessary to provide a comparator to detect the timing to switch from constant-current charge to constant-voltage charge for the protection IC. Moreover, it is necessary to provide a terminal to send the signal and a terminal to receive the signal for the protection IC and charge control IC.
In a conventional charge controller including a protection IC and a charge control IC, the protection IC has a function of turning off the charge control FET 13 at a predetermined overcharge detection voltage (about 4.275 V in the case of a lithium-ion battery) to protect a secondary battery from overcharge. On the other hand, the charge control IC switches from constant-current charge to constant-voltage charge at a predetermined voltage (about 4.2 V in the case of a lithium-ion battery) as described above. The switching of the controls is performed in such a manner that each IC uses a comparator to compare reference voltages with the battery cell voltage or terminal voltage of the battery pack to detect the timing to switch.
If the protection circuit and the charge control circuit are composed of different ICs, the reference voltages generated by the reference voltage generating circuits provided for the respective ICs are varied in opposite directions in some cases because of variations in manufacturing process. In such case, if the reference voltages of each IC are set to values close to each other, and the reference voltage of the protection IC is shifted to a lower value and the reference voltage of the charge control IC is shifted to a higher value, the ranges of detection voltages varied because of the shifts of the reference voltages overlap each other in some cases. The variation in each reference voltage is individually about +/−0.025 V.
When the overcharge detection voltage of the protection IC is lower than the constant-voltage charge switching voltage of the charge control IC, the protection IC detects that the battery voltage exceeds the overcharge detection voltage and turns off the charge control FET before the charge control IC detects that the battery voltage exceeds the constant-voltage charge switching voltage. Accordingly, the charge control IC cannot acknowledge that the charge control FET is turned off, thus causing a situation whereby the charge is not completed.
Accordingly, in a charge controller of a conventional lithium-ion battery, as shown in FIG. 6A, for example, the overcharge detection voltage Vdet2 of the protection IC is set to 4.275 V, and the constant-voltage charge switching voltage Vdet1 of the charge control IC is set to 4.200 V. This can provide a margin of about 0.02 V so that the variation ranges of reference voltages do not overlap each other even if the reference voltages vary. Herein, the overcharge detection voltage Vdet2 of the protection IC is set to 4.275 V because the charge inhibit region of lithium-ion batteries is equal to or more than 4.3 V and the variation of the reference voltage on one side which is 0.025 V is subtracted from the above voltage (4.3 V).
However, when the overcharge detection voltage Vdet2 of the protection IC and the constant-voltage charge switching voltage Vdet1 of the charge control IC have a large 0.075 V difference, the charge control IC often terminates charge control before the battery is actually fully charged, thus causing a problem of reduction in apparent battery capacity.
Moreover, it is known that while being charged, secondary batteries generate heat. When the amount of generated heat increases to raise the battery cell temperature, charge current is controlled according to the temperature for protecting the battery cell.
In a conventional battery charger which includes a protection IC and a charge control IC and is provided for a mobile phone or the like, for example, a thermistor is provided for the battery pack. In the battery charger, a signal according to change in temperature of the thermistor is sent to the charge control IC provided in the main device, and as the battery cell temperature rises or falls, control such as reducing charge current or reducing charge voltage is performed. In another case, the protection IC is provided with an element within the chip, the element changing with temperature in characteristic, such as a resistance, and based on the change in characteristic of the element, the overcharge detection voltage or the over-current detection voltage are changed so that the protection point is shifted.
However, if the charge control IC and the protection IC are configured to change the controls according to the respective conditions of the different temperature detection elements, the temperature detection elements have different temperature characteristics and different variations thereof.
As shown in FIG. 7A, therefore, the detection values by the protection IC concerning the overcharge protection and the over-current protection are deviated from the current-control switching voltage CV and the detection value concerning over-current protection by the charge control IC. If the charge control IC detects over-current prior to the protection IC, for example, the charge is stopped even in a chargeable temperature range, and the apparent battery capacity is reduced in some cases.
Moreover, if the protection IC detects the overcharge prior to the charge control IC and turns off the charge control FET, for example, the charge control IC cannot acknowledge that the charge control FET is turned off and could determine that the battery has failed, continue the control to apply charge current, or stop charging even in the chargeable temperature range.
FIG. 7A shows the relationship between temperature adjustment of the detection voltage concerning overcharge protection in the protection IC and temperature adjustment of the voltage CV for switching from constant-current charge control to constant-voltage charge control in the charge control IC. FIG. 7B shows the relationship between temperature adjustment of the detection current concerning overcharge protection in the protection IC and temperature adjustment of charge control current CC in the charge control IC. In FIGS. 7A and 7B, the horizontal axis indicates temperature, and the temperature increases from left to right.